Electroosmotic devices

ABSTRACT

Electroosmotic (EO) devices are provided which are not subject to mechanical wear and tear and with no moving parts, and having improved flow rates and electrical properties. Atomic layer deposition can be used to prepare three electrical terminal active zeta potential modulated EO devices from porous membranes. First, second, and further thin layers of materials can be formed with the pores. Thus, embedded electrodes can be formed along the length of the pores. The zeta potential in the pores can be modified by use of a voltage potential applied the embedded electrode, thereby achieving active control of surface zeta potential within the pores and active control of flow through the pores.

RELATED APPLICATIONS

This application claims priority to PCT patent application No. PCT/US10/32316 for “ELECTROOSMOTIC PUMP”, filed Apr. 23, 2010, which claims priority to U.S. provisional application Ser. No. 61/172,632, filed Apr. 24, 2009, both of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to non-mechanical micropump devices, and more particularly to electroosmotic devices.

BACKGROUND

Over the past decade, increasingly more research and development has gone into the development of lab-on-a-chip (LOC) systems. LOC systems are minute chemical processing plants, typically including a complex network of micro/nanoscale channels. In general, such LOC systems are configured for automatically performing common laboratory procedures, such as filtration, mixing, separation, and detection. LOC systems are of great interest in biomedical, pharmaceutical, environmental, and security applications, as they can potentially provide a fast, inexpensive, and portable means of handling and analyzing materials.

Of particular interest for LOC and other microfluidic systems is how to provide a robust and reliable means of transferring materials within the system. Although a variety of micropumps have been developed for LOC applications, electroosmotic (EO) pumps have garnered a significant amount of interest. EO pump devices are typically fabricated using macroporous or nanoporous materials to form an EO pump membranes. In general, EO pump devices are preferred in microfluidic systems, since they enable fluid pumping and flow control without the need for mechanical pumps or valves. Further, EO pump devices typically can minimize sample dispersion effects.

However, the relatively small sizes required for the pores in such EO pump devices generally result in a non-trivial fabrication process. Further, the performance of most conventional EO pump devices is generally limited. For example, some of the drawbacks of such conventional EO pump devices typically include high operating voltage requirements (e.g., on the order of 1 kV to 10 kV), electrolysis of water, oxidation of electrode surfaces, and Joule heating. Such drawbacks therefore limit the usefulness of conventional EO pump devices for certain applications. For example, high operating voltages typically required for conventional EO pump devices generally prevent EO pump devices from being successfully integrated into lab-on-a-chip (LoC) type portable devices.

SUMMARY

Embodiments of the invention concern electroosmotic pumps and methods for manufacture and use thereof. In a first embodiment of the invention, an electroosmotic device is provided. The device includes a substrate comprising at least one pore extending from a first major surface of the substrate to a second major surface of the substrate, where the at least one pore is one of a macropore, a micropore, or a nanopore. The device also includes at least one first material disposed on at least the inner surface of the at least one pore, where the at least one first material is electrically conductive. The device further includes at least one second material disposed at least on the at least one first material in the at least one pore, where the at least one second material is an electrical insulator. The device also includes at least one anode electrode adjacent to the first major surface and at least one cathode electrode adjacent to the second major surface.

In a second embodiment of the invention, a method of preparing an electroosmotic device is provided. The method includes obtaining a substrate comprising a first major surface, a second major surface, and at least one pore extending from the first major surface to the second major surface, where the pore is one of a macropore, a micropore, or a nanopore. The method also includes forming at least one first material on at least the inner surface of the at least one pore, where the at least one first material is electrically conductive. The method further includes depositing at least one second material on at least one first material, where the at least one second material is an electrical insulator. The method additionally includes providing at least one anode electrode adjacent to the first major surface and at least one cathode electrode adjacent to the second major surface.

In a third embodiment of the invention, a method for using an electroosmotic device is provided. The method includes providing an electroosmotic device comprising a substrate having at least one pore that is a macropore, a micropore, or a nanopore, at least one first material formed on the inner surface of the at least one pore, at least one second material disposed on the at least one first material, at least one anode electrode adjacent to a first major surface of the substrate, and at least one cathode electrode adjacent to a second major surface of the substrate, where the first material is electrically conductive and the second material is an electrical insulator. The method also includes applying a first voltage across the anode and cathode to generate electroosmotic flow through the at least one pore and applying a second voltage independently biased from the first voltage to the at least one first material to modify the electroosmotic flow.

In a fourth embodiment of the invention, an electroosmotic device is provided. The device includes a substrate comprising at least one pore extending from a first major surface of the substrate to a second major surface of the substrate, the at least one pore comprising one of a macropore, a micropore, or a nanopore. The device also includes plurality of dopant atoms embedded into at least the inner surface of the at least one pore to define at least one first material at the at least the inner surface of the at least one pore, the at least one first material being electrically conductive. The device further includes at least one second material disposed at least on the at least one first material in the at least one pore, where the at least one second material is an electrical insulator. The device additionally includes at least one anode electrode adjacent to the first major surface and at least one cathode electrode adjacent to the second major surface.

In the various embodiments, the cathode and anode electrodes can be formed using electrically conductive materials deposited on the substrate or an electrically conductive mesh adjacent to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a section of an electroosmotic device including a pore for directing fluid from a first reservoir or inlet to a second reservoir or inlet that is useful for describing the various embodiments of the invention;

FIG. 2 is a schematic of an exemplary microfluidic system including an EO micropump configured in the accordance with an embodiment of the invention;

FIG. 3 is a schematic of a portion of section of an electroosmotic device configured in accordance with the various embodiments of the invention;

FIG. 4 is a schematic illustration of an exemplary process flow for manufacturing EO pump devices in accordance with the various embodiments of the invention;

FIGS. 5A and 5B show top-down and cross-section SEM images, respectively, of a porous anodic aluminum oxide (AAO) substrate after ion milling;

FIGS. 6A and 6A are SEM images of the front side and back side, respectively, of a silicon substrate-based porous membrane;

FIG. 7 is an X-Y plot of sheet conductance as a function of ALD growth temperature;

FIG. 8A is an X-Y plot of sheet conductance as a function of the number of ALD cycles for as-deposited and annealed platinum thin films deposited using an ALD process at 300° C.;

FIG. 8B is an X-Y plot of resistivity of the films in FIG. 8A as a function of thickness;

FIGS. 9A, 9B, 9C, 9D, and 9E show deposition of Pt by ALD at 300° C. for 50 cycles, 100 cycles, 200 cycles, 400 cycles and 1000 cycles, respectively;

FIGS. 10A, 10B, and 10C show SEM images of Pt thin films deposited on AAO membranes with using the conformal ALD deposition processes described above and for exposure times of 0 s, 10 s and 30 s;

FIG. 11 is an SEM image of platinum tubes fabricated in AAO membranes;

FIG. 12 is a top-down SEM image of a silicon substrate with ALD deposition of Pt;

FIG. 13A shows a cross-sectional SEM image of ALD (atomic layer deposited) zirconia coated AAO substrate;

FIG. 13B shows a corresponding EDS Zr mapping showing uniform distribution of zirconia throughout the entire thickness of the 60 μm AAO substrate in FIG. 13A.

FIGS. 14A, 14B, and 14C show a top-down SEM images of an AAO substrate, the AAO substrate with a thin film ALD coating of ZrO₂, and the coated AAO substrate after the AAO walls have been removed to show single ZrO₂ nanotubes, respectively; and

FIGS. 15A, 15B, 15C, and 15D show zeta potential measurements of ALD-deposited alumina, titania, zirconia, and silica films, respectively, as a function of solution pH for different aqueous solutions (1 mM, 10 mM, 100 mM, and 1M) of potassium chloride (KCl).

DETAILED DESCRIPTION

The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.

Embodiments of the invention provide methods for fabricating electroosmotic (EO) pumps and devices therefrom. In particular, EO pump devices are provided consisting of a macroporous, microporous, or nanoporous membrane. Such EO pump devices are configured with electrodes on the surfaces of the membrane in order to provide a relatively high. electric field for inducing electroosmotic flow through the pores with relatively low voltages. Further, such EO pump devices are configured to include embedded electrodes in the pores, where the embedded electrodes can be independently biased relative to the surface electrodes. These embedded electrodes can then be used to adjust a surface zeta potential in the pores, and thus adjust the amount of flow and/or the direction of the flow in the pores.

Prior to describing the various exemplary embodiments of the invention, it is useful to provide a description of the role of the surface zeta potential in electroosmotic devices. This is illustrated with respect to FIG. 1. FIG. 1 is a schematic of a section of an electroosmotic device 100 including a pore 102 for directing fluid from a first reservoir or inlet 104 to a second reservoir or inlet 106. The device 100 can include anode electrodes 108 and cathode electrodes 110. The pore 102 consists of an opening in a membrane 112 with dielectric surfaces 114.

In general, the chemical equilibrium between a solid surface (e.g., surfaces 114) and an electrolyte solution typically leads to an interface region 116 acquiring a net fixed electrical charge due to the formation of a layer of mobile ions known as an electrical double layer (EDL) or Debye layer. In general, the formation of the EDL is a function of the surface zeta potential along the interior surfaces of the pore. Surface zeta potential refers to the electrical potential at an electrokinetic plane of shear at a solid surface in contact with a liquid and is generally a property of the materials forming the solid surface. During EDL formation, the charge due to surface zeta potential causes the local free ions in the liquid to rearrange and provide the region 116 with a nonzero net charge density near the solid-liquid interface. The interface region can consist of a stern region 116 a adjacent to surfaces 114 and having a large concentration of ions from the electrolyte and oppositely charged with respect to the surface zeta potential and a diffuse region 116 b with a lower concentration of such ions.

When an electric field is applied (e.g., via electrodes 108 and 110) to the fluid and across the length of the pore 102, the ions associated with the net charge in the interface region 116 are induced to move by the resulting Coulomb force. The direction of flow is dictated by the polarity of the surface zeta potential, as the ions accumulated in the interface region will have a polarity to that of the surface zeta potential. The viscous drag caused by movement of these ions localized in the interface region 116 transfers momentum to the rest of the fluid, including the fluid in a bulk region 118 (i.e., a region with no net charge). Since there is no other fixed surface inside the pore 102 against which the fluid in the bulk region 118 can dissipate this momentum, the fluid in the bulk region 118 also begins to move with the same velocity as the ions in the interface region 116. The resulting flow is termed electroosmotic flow.

Depending on the ionic concentration in the electrolyte, the EDL thickness can vary. For example, the EDL can be as small as 3 nm for a 1×10⁻² M electrolyte and up to 300 nm for deionized water (1×10⁻⁶ M). As a result, the channel hydraulic diameter and EDL thickness may become comparable in some pore configurations, depending on the pore size and the EDL thickness. If large ionic concentration buffers result in EDL thicknesses on the order of a few nanometers and the pore diameter is the order of 200 nm and above, the EDL constitutes a very small portion of the flow domain. This simplification is convenient for mathematically describing fluid flow in the pore 102. In particular, rather than modeling the flow using a Poisson-Boltzmann equation that governs the ion distribution near the surfaces and the corresponding EDL effects on momentum transport, the fluid velocity in vicinity of the charged surface can instead be modeled by the Helmholtz-Smoluchowski electroosmotic velocity:

$\begin{matrix} {U_{HS} = {- \frac{{\zeta ɛ}\; E_{x}}{\mu}}} & (1) \end{matrix}$

where ζ is the zeta potential, ∈ is permittivity of the liquid, μ is the dynamic viscosity, and E_(x) is the electric field applied in the stream wise direction. For steady electric field and constant channel cross-section, this equation also models the EO-flow velocity in the channel/pore, which happens to be a plug type flow. Without considering the EDL effects and/or upstream/downstream pressure head, the flow rate (Q) in a pore then be approximated by:

Q=U _(HS) ×A,  (2)

where A is the pore area. Thus, Equation 2 shows that the flow rate within a pore of a selected size can be increased either by increasing the zeta potential or the applied electric field.

As described above, the surface zeta potential is generally a function of the materials at the surfaces of the pore. However, the zeta potential will also vary as a function of the fluid, specifically its ionic strength and pH. Thus, it is possible to provide “passive” zeta potential control to improve flow through the pore by selection of an appropriate surface material for the pore based on the types of buffers to be used, or vice versa. Unfortunately, the issue with providing such “passive” zeta potential control is that once the pores are manufactured, it is not possible to provide further adjustment of the surface zeta potential other than by adjustment of the buffer. In many cases, this is undesirable or not possible. As a result, the flow can only be actively adjusted via adjustment of the applied electric field. Thus, to achieve some levels of flow, a relatively high electric field needs to be applied. However, such electric fields levels can result in electrolysis of water, oxidation of electrode surfaces, and Joule heating, as described above. As a result, many conventional EO pump are configured to provide relatively high flows for a limited combination of buffer types and voltages ranges.

Accordingly, the various embodiments of the invention overcome the limitations of such EO pump configurations by providing “active” control of the zeta surface potential. As described above, an EO pump in accordance with the various embodiments of the invention is configured to include embedded electrode portions adjacent to the inner surfaces of the pores. In operation, a voltage can be applied to the embedded electrode in addition to the voltages applied to the electrodes in the reservoirs. As a result, the applied voltage at the embedded electrode alters the effective electrical potential at the solid surface in contact with the liquid, altering the surface zeta potential with respect to the fluid. This therefore allows “active” adjustment of the surface zeta potential and provides an additional means for adjusting flow through the pore. Consequently, since the embedded electrode can be used to alter the surface zeta potential, the adverse effects of applying a high electric field can be reduced or eliminated by applying lower electric fields, but providing an increased flow by altering the surface zeta potential via the “active” control of the surface zeta potential.

Referring now to FIG. 2, there is shown a schematic of an exemplary microfluidic system 200 including an EO micropump 202 configured in the accordance with an embodiment of the invention. As shown in FIG. 2, the system 200 includes an EO micropump 202 consisting of a porous substrate or membrane 204 with first and second outer pore surfaces 205, 206 and electrodes 208 disposed on these surfaces. Further, the EO pump 202 is configured to include embedded electrodes (not shown), as described above. The configuration and fabrication of EO pump 202 will be described in further detail below with respect to FIG. 3.

In system 200, the electrodes 208 can be coupled to a direct current (DC) power supply 210 to provide a voltage across electrodes 208 and thus between the surfaces 205, 206 of micropump 202 to induce electroosmotic flow. For example, a voltage between 0 and 10V, such as 1-5V, can be provided. In some configurations, a multi-meter 212 can be provided to monitor the output of supply 210. Additionally, the system 200 can include a second DC power supply 211 to generate a voltage for the embedded electrodes (not shown) in EO pump 202. However. The various embodiments are not limited in this regard and a single voltage supply or voltage supply system can be used to provide voltages for electrodes 208 and the embedded electrodes in EO pump 202.

The EO micropump 202 can disposed in a membrane holder 214 coupled to an inlet 216 and an outlet 218. The EO micropump 202 can be arranged in membrane holder so as to provide a fluid connection between inlet 216 and outlet 218. Inlet 216 can be coupled to a first reservoir 220 and outlet 218 can be coupled to a second reservoir 222. Additionally, reservoir 220 can be associated with a first scale 224 and second reservoir 222 can be associated with a second scale 226. Thus the flow rate between reservoirs 220 and 222 can be monitored by the changes at scales 224 and 226, respectively.

In some configurations, pressure transducers 228 can be provided at the inlet 216 and the outlet 218 for measuring the pressure across the pump 202. Additionally, an outlet valve 230 can be provided to prevent flow from reservoir 220 to 222. Alternatively, valve 230 can be used with transducers 228 to measure the maximum pressure build-up across the pump 202 by closing the outlet valve 230.

Referring now to FIG. 3, there is shown a schematic of a portion of section of an electroosmotic device 300 configured in accordance with the various embodiments of the invention. For example, device 300 can be used to provide the EO pump 202 in FIG. 2. As shown in FIG. 3, device 300 includes a membrane 312 having one or more pores 301 for defining one or more channels 302 for directing fluid from a first reservoir or inlet portion 304 to a second reservoir or inlet portion 306. The device 300 can also include anode electrodes 308 and cathode electrodes 310 for inducing electroosmotic flow through the channels 302 by applying a voltage across electrodes 308 and 310. The membrane 312 can also have dielectric surfaces 314 formed thereon, similar to the configuration in FIG. 1, defining the inner surface of the channels 302. Thus, similar to the configuration illustrated in FIG. 1, the resulting zeta potential at the dielectric surfaces 314 can result in the formation of an interface layer 316 with a stern region 316 a and a diffuse region 316 b.

In contrast to the configuration in FIG. 1, the device 300 can be configured to include embedded electrode portion 320 that is coupled to a separate voltage supply (not shown). As described above, by applying a voltage at the embedded electrode 320, the effective surface zeta potential can be modified. In particular, the net charge or effective zeta potential provided at the surface 314 is interface provided by the combination of the intrinsic zeta potential for the electrolyte/surface material combination in use and the potential at the embedded electrodes. Thus, by varying this effective zeta potential, the total number or concentration of ions accumulating in the stern region 316 a and the diffuse region 316 b of the interface layer 316 can be adjusted. Further, the thickness of the interface layer 316 can also be adjusted. Accordingly, as the ion concentration and interface layer thickness is adjusted in these regions, the impact of the Columbic forces is modulated. Accordingly, the flow through the channels 302 can be adjusted upwards or downwards. In some instances, by providing a voltage opposite in polarity to the polarity of the intrinsic zeta potential of the dielectric surfaces 314, the flow can be halted or the direction of flow can be reserved.

The device 300 can therefore be operated in two modes. In a first “passive” mode, the device 300 can rely on the intrinsic zeta potential by allowing the embedded electrode 320 to remain floating or to be coupled to ground. In a second “active” mode, the electrode 320 can be biased to modulate the surface zeta potential and thus modulate the flow of liquid through the channels 302 to increase, decrease, or reverse flow.

Manufacture of EO Pump Devices

Referring now to FIG. 4, there is shown a schematic illustration of an exemplary process flow 400 for manufacturing EO pump devices in accordance with the various embodiments of the invention. The process 400 begins at step 402, where the membrane 312 with one or more pores 301 is provided. In the various embodiments of the invention, the membrane or porous substrate can be macroporous, microporous, or nanoporous. These porous membranes can be fabricated using substrates made of inorganic materials such as, for example, silicon or alumina. Further details regarding the fabrication of some exemplary membrane configurations will be described in greater detail below.

In the membranes, the average pore size can be between approximately 200 nm and 10 μm. For example the average pore size can be 10 μm, 5 μm, or 1 μm. Further the range of pore sizes can vary. For example, in some embodiments, the pore size can be, for example, about 200 nm to about 10 μm, about 200 nm to about 5 μm, or about 200 nm to about 3 μm. Further, the aspect ratio of the pores 301 can vary. For example, in some embodiments the aspect ratio can vary from 5 to about 1,200, such as from approximately 300 to about 1200. However, the various embodiments of the invention are not limited in this regard and aspect ratios of 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 1000 can also be provided. These pore sizes and aspect ratios are provided for illustrative purposes only. Accordingly, a membrane in accordance with the various embodiments of the invention can be manufactured with other pore sizes or aspect ratios.

Once the membrane 312 is provided or fabricated at step 402, the method 400 can then proceed to step 404. At step 404, the embedded electrodes 320 can be formed.

In some embodiments, the embedded electrodes 320 can be formed by depositing a layer of electrically conductive material on at least inside the inner surfaces of the pores 301 in the membrane 312. In such embodiments, the embedded electrodes 320 can be formed in a variety of ways using a variety of electrically conductive materials so that the embedded electrodes are formed 320, but that the resulting channels 302 are not significantly reduced in size or closed off.

In these embodiments, the electrically conductive material can consist of a metal, a conducting metal nitride, a semiconductor, or a conducting metal oxide. Non-limiting examples of metals and metal nitrides include Ti, Au, Pt, Al, Cu, Ag, and metal nitrides, such as TiN, thereof or any other stable noble metal. A non-limiting example of a semiconductor includes ZnO. Such materials can be deposited using atomic layer deposition (ALD), chemical vapor deposition (CVD), or any other methods suitable for conformal deposition of electrode materials over all surfaces, including pore surfaces, of the microporous or nanoporous membrane geometries described above. Some exemplary methods for such deposition will be described below in greater detail.

In other embodiments, the embedded electrode 320 can be formed by increasing the electrical conductivity of the membrane 312 along at least the inner surfaces of the pores 301. For example, the membrane 312 can be formed from a semiconducting material and at least the inner surfaces of the membrane 312 can be exposed to dopants to increase the electrical conductivity of at least these inner surfaces of the membrane to form the embedded electrode 320. In such embodiments, the dopants can be provided via diffusion or ion implantation processes. For example, in the case of membrane 312 comprising silicon or another column IV semiconductor material, an N+ or P+ layer can be formed to define the embedded electrode 320. However, these embodiments are not limited solely to column IV materials. Rather, membrane 312 can be formed from any type of semiconductor material. Further, the various embodiments are not limited to any particular diffusion or implantation techniques. Rather, any other types of doping techniques can be used in the various embodiments. Some exemplary methods for such deposition will be described below in greater detail.

After the embedded electrodes are formed at step 404, these materials can be electrically isolated from the channel 302 at step 406. In particular, the dielectric material defining the surface dielectric layer 314 can be deposited over substantially all the surfaces of the membrane in which an embedded electrode 320 was formed at step 404. As a result, an embedded electrode 320 that is electrically isolated from the interior of pore 302 is formed.

In some embodiments, the second insulating material comprises a semiconductor oxide or an oxide or insulating metal oxide. Non-limiting examples of semiconductor oxides or metal oxides include HfO₂, ZrO₂, Al₂O₃, TiO₂, and SiO₂ or any other electrically insulating material.

Once the surface dielectric materials are formed at step 406, the method can proceed to step 408 to form electrodes 308 and 310.

In some embodiments, an electrically conductive material can be deposited at step 408 on the outer surfaces of the membrane 312. The deposition can be configured to avoid deposition of such materials in the channels 302. For example, sputtering or e-beam techniques can be used to deposit such materials. However, any other methods that result in preferential deposition of materials on the outer surfaces of the membrane can be used without limitation. Accordingly, layers of materials forming electrodes 308 and 310 are primarily confined to regions outside the pores. In some embodiments, the electrodes 308 and 310 comprise a metal. Examples of useful metals, include, but are not limited to, Au, Pt, and W, and any other stable noble metals or alloys thereof. In other embodiments, other metals and alloys, metal nitrides, semiconductors, semiconductor oxides, or metal oxides can also be used without limitation. Processes for forming such materials will be described below in greater detail.

In other embodiments, the electrodes 308 and 310 can be formed by disposing an electrically conductive mesh on or above each of the outer surfaces of the membrane 312. In such configurations, the mesh can be an electrically conductive substrate or film with perforations or other openings to allow fluids to traverse through the mesh without significantly impacting flow through the channels 302. In other configurations, the perforated substrate or film can be electrically non-conductive with a layer of electrically conductive material formed thereon. Processes for forming such a mesh will be described below in greater detail.

Additionally, the process flow described above can include additional steps not shown in FIG. 4. For example, additional photolithography and etching steps can be provided for forming an electrical contact region for any of electrodes 308, 310, or 320. Further, additional photolithography or wirebonding techniques can also be included in such methods to provide structures for electrically coupling any of electrodes 308, 310, and 320 to a DC voltage supply. In another example, following step 408, a passivation layer can be deposited over the electrodes 308 and 310 or over any contact regions formed for device 300. For example, a layer of dielectric material can be deposited. Such passivation layers can be used to passivate the exposed surfaces of electrodes 308 and 310 or any other reactive materials exposed to the liquid in device 200. Further, a process flow in accordance with the embodiment of the invention can include any additional steps required for configuring device 300 to operate in the system illustrated in FIG. 2 or any other electroosmotic system requiring an EO pump.

The coaxial nanostructures described above (i.e., the embedded electrodes 320 and surface dielectric layer 314) can be prepared, as noted above, according to various methods.

A. Porous Membrane

In some embodiments of the invention, the porous membrane can be a nanoporous anodized aluminum oxide (AAO) film. A nanoporous AAO substrate can be prepared by a two-step anodization procedure. First, high purity aluminum sheets (˜0.5 mm thick) can be degreased using, for example, acetone or any other suitable solvent. These sheets can then be electropolished in a solution of HClO₄ and ethanol. For example, an exemplary process can be performed using a 1:4, v/v solution at 20 V for 5-10 min or until a mirror-like surface is achieved.

Thereafter, a first anodization step can carried out to form a porous alumina layer on the surface of the sheet. For example, an exemplary process can include anodization in a 0.3 M oxalic acid solution electrolyte under a constant direct current (DC) voltage of 80 V at 17° C. for 24 h. After the first anodization step, the porous alumina layer can be stripped away from the Al substrate. For example, the stripping can be performed by treating the anodized sheet using a solution containing 6 wt % phosphoric acid and 1.8 wt % chromic acid at 60° C. for 12 h. A second anodization step can then be carried out. For example, the Al sheet can be treated using a 0.3 M oxalic acid solution under a constant direct current (DC) voltage of 80 V at 17° C. for 24 h. Finally, AAO substrates with highly ordered arrays of pores can be obtained by selectively etching away the unreacted Al and, if necessary performing planarization of the AAO substrate. For example, the unreacted Al can be etched away using a saturated HgCl₂ solution. The planarization can then be performed using ion milling.

A result of the above-mentioned processes is illustrated in FIGS. 5A and 5B. FIG. 5A shows a top-down scanning electron microscope (SEM) image of the pore structure of a 60 μm thick, planarized AAO substrate. As shown in FIG. 5, the resulting pore size is in the range of 200-300 nm and the wall width between pores is approximately 50 nm. As can be observed in FIG. 5, some of the pores are connected via thinning of the wall. FIG. 5B is a cross-sectional SEM image of the AAO substrate shown in FIG. 5A. As shown in FIG. 5B, the pores are substantially parallel to each other. The inset in FIG. 5B shows the formation of branches in some of the pores. Such branches can be eliminated by using shorter anodization times, although this results in a shorter pore length.

In some AAO formation processes, one end of the pores can have a smaller pore size than another end of the pores, to a depth of a few micrometers. Accordingly, in some processes, this thin layer can be removed by etching.

High magnification FE-SEM of such samples also shows a smooth morphology of the inside walls of the AAO pores can be achieved. Such smooth wall formation is generally desirable, as the process using for forming subsequent layers in the pores replicate the surface of the pores in the membrane on an Angstrom scale.

However, the various embodiments of the invention are not limited solely to AAO-based porous membranes. In other embodiments, as described above, the porous membranes can be formed using silicon or other types of semiconductor substrates. In such a process flow, openings can formed in the semiconductor substrate via photolithography processes followed by substrate etching processes. Accordingly, this process flow can include deposition and removal of photoresist, deposition and etching of masking layers, and cleaning or degreasing steps. The advantage of such a photolithography-based process is that the size and spacing of the pores can be directly controlled. Accordingly, the ordered porous membranes can be more reliably and reproducibly manufactured. For example, the result of such a process is shown in FIGS. 6A and 6B.

FIGS. 6A and 6A are SEM images of the front side and back side, respectively, of a silicon substrate-based porous membrane. As shown in FIGS. 6A and 6B, the pores are substantially identical and arranged in a regular array. Further, since more control over size and spacing of the pores is provided, issues regarding branching or connected pores are eliminated. Additional, the etch process or post-etch processes can be configured to provided relatively smooth surface in the pores.

Although various exemplary processes have been described above, the various embodiments of the invention are not limited in this regard. Rather, one of ordinary skill in the art will readily recognize that other process can be used to form AAO or silicon-based porous membranes in accordance with the various embodiments of the invention.

B. Embedded Electrode

In the various embodiments of the invention, a principal concern is the deposition process for the electrically conductive materials forming the embedded electrodes in the pores. In particular, to provide superior performance, the electrically conductive materials should be substantially uniformly deposited along the length of the pores, having a relatively smooth surface morphology, and high sheet conductance (i.e., low sheet resistance). As used herein with respect to any comparison, the term “substantially” refers to a difference of less that 20%.

In some embodiments of the invention, such embedded electrodes can be formed using platinum thin films deposited by atomic layer deposition (ALD). However, the various embodiments are not limited solely to ALD. For example, methods can also include chemical vapor deposition (CVD) techniques or any other methods available to deposit layers (also referred to as films herein) of the types of materials described above on the inner surface of the pores of a nanoporous substrate.

Briefly, ALD technology deposits thin films using pulses of chemical precursor gases to adsorb at the target surface one atomic layer at a time. ALD is based on the sequential deposition of individual monolayers or fractions of a monolayer in a controlled fashion. More specifically, in ALD the growth substrate surface is alternately exposed to the vapors of one of two chemical reactants (complementary chemical precursors), which are supplied to the reaction chamber one at a time. The exposure steps are separated by inert gas purge or pump-down steps in order to remove any residual chemical precursor or its by-product before the next chemical precursor can be introduced into the reaction chamber. Thus, ALD involves a repetition of individual growth cycles. Since a film deposited by ALD is grown in a layer-by-layer fashion and the total film thickness is given by the sum of the number of ALD cycles, it is possible to calculate the number of cycles necessary to reach a desired final film thickness. Conversely, the thickness of a film can be set digitally by counting the number of reaction cycles. In general, ALD achieves deposition rates on the order of 0.1-1.0 Å per cycle and with cycle times ranging from one to ten seconds. Due to the self-limiting nature of the surface reactions, accidental overdosing with precursors does not typically result in increased film deposition. Thus, ALD is able to achieve conformal films with good across-wafer film thickness uniformity and good step coverage. Because of the nature of ALD, film thickness is immune to variations caused by non-uniform distribution of reactant vapor or temperature in the reaction chamber. Further, a variety of chemical precursors may be used with ALD, depending upon the desired film. Specific chemical precursors are provided in the Examples below. For example, platinum thin films can be formed using an ALD process with (methylcyclopentadienyl) trimethylplatinum (MeCpPtMe₃) and oxygen (O₂) as the precursors. The deposition temperature for such a process can be in the range of 270-320° C. to provide a predictable sheet conductance, as shown in FIG. 7.

FIG. 7 is an X-Y plot of sheet conductance as a function of ALD growth temperature. As shown in FIG. 7, the conductance (and thus resistivity) of a ALD platinum thin film approaches a constant value when stable ALD Pt film growth is achieved. As noted above, this constant value is achieved for the above-mention process in the temperature range of 270-320° C. The increase in growth rate after 320° C. is indicative of Pt precursor decomposing. Further, a temperature in such a range can also be selected based on other criteria. For example, based on other properties such as surface roughness and impurity species in the film. Based on such properties and the data of FIG. 7, in at least one embodiment, an ALD deposition temperature of 300° C. can be used to provide stable growth of platinum thin films with acceptable surface roughness and impurity species.

As the sheet conductance, G_(sh)=t/ρ (where t is the film thickness and ρ is the resistivity) is proportional to film thickness, sheet conductance with have an approximately linear relationship to the number of ALD cycles. Accordingly, a number of ALD cycles to be used (and thus the film thickness) can be selected to provide a desired sheet conductance. This is illustrated in FIG. 8A.

FIG. 8A is an X-Y plot of sheet conductance as a function of the number of ALD cycles for as-deposited and annealed platinum thin films deposited using an ALD process at 300° C. As shown in FIG. 8A, good linearity is demonstrated between sheet conductance and ALD cycles, specifically after 300 cycles. This shows that the above-mentioned ALD processes provide a substantially constant growth rate for Pt thin films. As noted above, the Pt thin films were also annealed. Specifically, the films were annealed in forming gas (95% N2 and 5% H2) at 450° C. for 30 min. The forming gas anneal (FGA) was effective to improve the adhesion and conductivity of the Pt thin films by passivating dangling bonds in the grain boundaries with hydrogen. As also shown in FIG. 8A, a higher sheet conductance was observed after FGA.

However, as shown in FIG. 8A, there is a drop in the sheet conductance for both as-deposited and annealed Pt films with ALD cycles less than 300. It is believed that the decrease in the sheet conductance or the increase in the sheet resistance is either due to the surface scattering or the structure of Pt film when the film is thinner than a certain thickness. For example, as shown in FIG. 8A, for the first 150 cycles, no sheet conductance was observed. This may be because no continuous Pt film was yet formed.

The resistivity of these Pt thin films is shown in FIG. 8B. FIG. 8B is an X-Y plot of resistivity of the films in FIG. 8A as a function of thickness. For FIG. 8B, the resistivity values were calculated using the film thickness as determined by cross-sectional TEM. As shown in FIG. 8B, the resistivity of these films was shown not to be strongly dependent on thickness for films thicker than 15 nm. The 20 nm thick Pt film after anneal showed a resistivity of 12 μΩ-cm, which is very close to the bulk resistivity of Pt (10.8 μΩ-cm). Thus an ALD-deposited Pt film of 15 nm is thick enough to achieve good electrical conductivity.

The growth of the Pt thin films grown using the above-mention ALD processes were analyzed by cross-sectional TEM (XTEM) to determine surface morphology and structure over time. This is shown in FIGS. 9A-9E. FIGS. 9A, 9B, 9C, 9D, and 9E show deposition of Pt by ALD at 300° C. for 50 cycles, 100 cycles, 200 cycles, 400 cycles and 1000 cycles, respectively. FIG. 9A shows the very early stage of Pt deposition on a Si substrate with native oxide. As shown in FIG. 9A, no continuous Pt thin film was formed. Rather, isolated Pt nanoparticles (size less than 5 nm) are randomly distributed on the surface.

As shown in FIG. 9B, with more deposition cycles, the Pt nanoparticles continue to grow. In certain areas, the Pt islands coalesce into larger, but still isolated islands, as shown in FIG. 9B. With addition cycle, as shown in FIG. 9C, the larger Pt islands coalesce, thereby forming a continuous Pt network. However, at this stage, the Pt film may not have covered the whole surface and may be a porous surface with low sheet conductance. Thus, this may explain the relatively low sheet conductance corresponding to 200 ALD cycles observed in FIG. 8A.

As additional ALD cycles are performed (>300), the Pt film becomes thicker and continuous Pt, with a smooth surface, as shown in FIGS. 9D and 9E. At this point, the ALD growth follows the classical model of monolayer by monolayer growth per ALD deposition cycle. Based on the TEM images, it is determined that astable growth rate 0.5 Å/ALD cycle can achieved for the precursors described above at or around 300° C.

FIGS. 10A, 10B, and 10C show SEM images of Pt thin films deposited on AAO membranes with using the conformal ALD deposition processes described above and for exposure times of Os, 10 s and 30 s. The pore size and thickness of the AAO membranes were 250-300 nm and 60 μm, respectively, which correspond to an aspect ratio of more than 200. FIG. 10A shows that the Pt can be deposited 10 μm deep into the AAO pore without exposure time, with possible thickness gradient from the surface. The bright area shown the regions coated with Pt deposition. The presence of Pt was confirmed by energy dispersive spectroscopy (EDS) mapping. FIG. 10B shows that the penetration of Pt is deeper with a 10 sec exposure. However, FIG. 10C shows that for further exposure times, the depth penetration into the AAO membranes saturates at around 20 μm from the surface, even with exposure times of 30 sec. This is attributed to the combined effects of AAO pore size and the Pt precursor diffusion rate, which is inversely proportional to the square root of the molecular weight for this Pt precursor.

The quality of such platinum films is shown in FIG. 11. FIG. 11 is an SEM image of platinum tubes fabricated in AAO membranes. In FIG. 11, the AAO membrane has been etched away using a NaOH solution. The SEM image reveals that the Pt tubes are about 15 μm in length. This'length is less than the Pt penetration depth shown in FIG. 9C with 30 sec exposure time. However, this can be explained by the TEM inset images (a) and (b) in FIG. 11. Inset (a) is a TEM image of the portion of a Pt tube formed near a surface of the AAO membrane. Inset (b) is a TEM image of the portion of the Pt tube formed away from the surface of the AAO membrane. As show in inset (a), at the end close to the surface of the AAO membrane, the Pt film is continuous and forms dense tubes. In contrast, away from the surface of the AAO membrane, the Pt film is discontinuous. Accordingly, the processes described above will be suitable for fabrication EO pump devices with AAO membrane thicknesses on the order of 15-20 nm. However, other membrane thicknesses can be used by adjusting the above-mentioned processes.

To provide EO pump devices with longer pore lengths and using the above mentioned ALD processes, the pore size can be increased. For example, although control of pore size and distribution is somewhat constrained with respect to AAO membranes, using silicon or semiconductor substrates allows greater control of pore size and distribution, as described above.

For example, as shown in FIGS. 6A and 6B, a silicon substrate can be subjected to provide an array of openings defining pores, where the openings are ˜1 μm wide and ˜1 μm apart. In such a configuration, the significantly larger openings allow the precursor for the ALD Pt deposition processes described above to reach the entire length of the openings and thus provide uniform deposition along the entire length to form Pt tubes extending along the entire length of the pores. This is illustrated in FIG. 12. FIG. 12 is a top-down SEM image of a silicon substrate with ALD deposition of Pt. For purposes of FIG. 12, a portion of the silicon substrate has been etched away to show the Pt tubes. These tubes extend along the entire length of each of the openings in the substrate.

In other embodiments, as described above, the embedded electrode layer can be formed by incorporating dopants into the surface of the membrane, at least along the inner surfaces of the pores, to provide a surface layer with high electrical conductivity. For example, in one exemplary process, the embedded electrodes can be formed by diffusion doping a porous silicon membrane with n-type doping so that the surface of each pore is rendered n+ and thereby rendered highly electrically conductive. Such a method can use gas phase doping of the surface layer of the porous silicon membrane, using phosphine or arsine gas. Gas phase diffusion is advantageous for such a process since a gas can easily penetrate the pores in the Si membrane and diffuse into the exposed silicon surface. Further, the depth of the diffused layer and the diffusion dose can be precisely controlled by adjusting diffusion time and temperatures during and after exposing the silicon membrane to the doping gas. However, as described above, the various embodiments are not limited to silicon membranes and n+ doping. Rather, any type of semiconductor membrane can be used and any appropriate types of dopants can be used.

Although various exemplary processes have been described above, the various embodiments of the invention are not limited in this regard. Rather, one of ordinary skill in the art will readily recognize that other processes and materials can be used to form the embedded electrodes in accordance with the various embodiments of the invention.

C. Dielectric Surface Layer

As described above, the inherent zeta potential is based on the dielectric surface layer disposed over the embedded electrodes and the chemical properties of the liquid. Accordingly, selection of the materials of the dielectric surface layer can be selected to provide a desired level of “passive” control, as described above with respect to FIG. 1. Thereafter, the embedded electrode can be used to adjust the zeta potential and provide “active” control, also as described above.

As described above with respect to the embedded electrodes, a smooth surface morphology and low impurity levels are also desirable for these films. Further, a dielectric film that extends along an entire length of the pores is also desirable. Accordingly, in some embodiments of the invention, such films can also be provided using ALD processes.

For example, ALD transition metal oxide film ZrO₂ can be deposited at 250° C. using tetrakis (dimethylamido) zirconia and H₂O as precursors. FIG. 13A shows a cross-sectional SEM image of ALD (atomic layer deposited) zirconia coated AAO substrate. FIG. 13B shows a corresponding EDS Zr mapping showing uniform distribution of zirconia throughout the entire thickness of the 60 μm AAO substrate. Further, FIGS. 14A, 14B, and 14C show top-down SEM images of an AAO substrate, the AAO substrate with a thin film ALD coating of ZrO₂, and the coated AAO substrate after the AAO walls have been removed to show single ZrO₂ nanotubes, respectively. As shown in these figures, the ZrO₂ is capable of forming dense, smooth ZrO₂ dielectric layers.

Another example of a suitable dielectric film is ALD Al₂O₃ deposited at 300° C. using trimethylaluminum (TMA) as precursor and water vapor as oxidizing agent. In such a process, a precursor pulse time can be 0.1 sec for both precursors with a purging pump time was set at 5 sec. Another example is ALD TiO₂ deposited at 250° C. using titanium isopropoxide (Ti(iPrO)₄) and H₂O vapor as precursors. Still another example is silica formed using any available methods. However, the various embodiments of the invention are not limited to these dielectric materials and the described processes and other dielectric materials and/or processes can also be used.

FIGS. 15A, 15B, 15C, and 15D show zeta potential measurements of ALD-deposited alumina, titania, zirconia, and silica films, respectively, as a function of solution pH for different aqueous solutions (1 mM, 10 mM, 100 mM, and 1M) of potassium chloride (KCl). This data was collected using electrophoretic light scattering measurements. As shown in FIGS. 15A-15D, the molarity of the solution has a significant impact on surface zeta potential. In particular, the molarity and the zeta potential are inversely related. Further, as shown in FIGS. 15A-15D, the zeta potential is reduced with increasing PH.

The pH at which the transition from positive to negative occurs is called the isoelectric point. In other words the isoelectric point can be defined as the pH value of the aqueous solution, where the suspended particles or surfaces carry no net electric charge. It is commonly observed that solid surfaces charge to form a double layer when in contact with a liquid. For example, in the case of the aqueous KCl solution, the surface charge determining ions are H+ and OH−, from the water, and the added K+ and Cl− ions and the oxide films are assumed to be covered with OH⁻ surface hydroxyl groups. FIGS. 15A-15D also shown that the zero point charge (ZPC) remains approximately the same even though the KCl concentration is different. In a direct comparison the isoelectric point of ALD alumina (Al₂O₃) occurred at a pH value exceeding 8. Next is the isoelectric point of ALD zirconia (ZrO₂) at a pH value of around 7.5, while the zero point charge of titania (TiO₂) occurred at a pH value of about 6. In the case of silica, the isoelectric point occurs at a pH of about 3.

In all these cases, the maximum zeta potential values vary depending on the type of dielectric surface. Accordingly, depending on the level of control and the range of zeta potential values required, an appropriate dielectric material can be selected. Additionally, this selected can be further based on the molarity and pH values that will be observed. For example, the data in FIGS. 15A-15D suggests that an ALD alumina surface film is the better candidate for microfluidic applications that require high, positive polarity zeta potentials. In contrast, silica surfaces would be better for microfluidic applications that require high, negative polarity zeta potentials.

Although various exemplary processes have been described above, the various embodiments of the invention are not limited in this regard. Rather, one of ordinary skill in the art will readily recognize that other processes and materials can be used to form the dielectric layers in accordance with the various embodiments of the invention.

A description of methods and calculations for evaluating the performance of the disclosed electroosmotic pumps is provided below. In providing an EO pump, materials can be selected to be biocompatible and satisfy other commercialization requirements. Several embodiments can be described further for which other materials can be used and adapted for a particular application.

For example, in some embodiments of the invention, flow chambers can be built by sandwiching a single or multiple porous substrates within a PDMS microchannel and a millimeter-scale plexiglass (PMMA) channel. After curing the PDMS, the porous substrate can be installed. The integrated system can be sealed using an adhesive. In one design, space can be created for each nanoporous substrate within the two pieces of the PMMA channels. Thin PDMS spacers or plastic o-rings can be used as a bushing material for bolting the nanoporous substrates between the two sides of the PMMA channels.

The performance of such EO pumps can be evaluated based on the flow rate and thermodynamic efficiency, as outlined in Chen, et al., “Low-Voltage Electroosmotic Pumping Using Porous Anodic Alumina Membranes,” Microfluid Nanofluid, 5(2); 235-244, 2008. The thermodynamic efficiency (η_(eff)) is given by:

$\begin{matrix} {\eta_{eff} = \frac{\Delta \; P \times Q}{V \times I}} & (3) \end{matrix}$

where V is the applied voltage, and I is the electric current, ΔP is the pressure change across the membrane and Q is the volumetric flow rate. The numerator of this equation is the power delivered by the pump, while the denominator is the electrical power input to drive the EO pump. For a fixed pressure load, it is desirable to have the maximum flow rate per given electrical power.

In order to obtain the total mass flow rate from the nanoporous substrate, the formula derived by Yao, et al., “Porous Glass Electroosmotic Pumps: Design and Experiments,” Journal of Colloid and Interface Science, 268(1); 143-153, 2003 and Yao; S, and Santiago, J. G., “Porous Glass Electroosmotic Pumps: Theory,” Journal of Colloid and Interface Science, 268(1); 133-142, 2003 may be used:

$\begin{matrix} {{Q_{\max} = {\frac{{- \psi}\; A\; {\delta ɛ\zeta}\; E_{x}}{\mu} = {\psi \; A\; \delta \times U_{HS}}}},} & (4) \end{matrix}$

where ψ is the porosity, A is the membrane area, and δ is a correction parameter to account for the EDL displacement thickness. As noted above with respect to Equation 1, the maximum flow rate is related to the Helmholtz-Smoluchowski EO velocity UHS and ψ×A is the net flow area, while 6 is the correction for finite EDL effects on the flow rate, which is expected to be δ≈0.9 for a 1×10⁻²M electrolyte. The axial electric field E_(x) can be predicted by dividing the potential drop across the membrane (Veff) with the membrane thickness, L. Using a porosity value of ψ=0.3 (a typical value), zeta potential of 25 mV, nanoporous substrate area of 0.78 cm² (A=π×0.5²) and a target potential difference of 1 V across the L=50 μm thick nanoporous substrate, a maximum flow rate of 50 μL/min is calculated. Thereafter, using Equation 4, the maximum flow rate increases linearly with membrane porosity, zeta potential and the applied electric field. Porosities of about ψ=0.7 and zeta potential of ζ=80 mV using ALD covered silica nanoporous substrates will provide a 10 fold increase in the flow rate, resulting in Qmax=0.5 ml/min. Thus, the maximum value of Qmax/A/V for conventional EO pump devices (with electrodes in the reservoirs away from the pore openings) is generally about 0.15 ml/(minV cm²). The EO-pump shown in FIG. 1 (with electrodes at or near the pore openings) provides a Qmax/A/V of 0.3-0.6 ml/(min V cm²).

Calculations show that the performance of the three-terminal EO pump shown in FIG. 3 can provide an order of magnitude increase in the maximum Qmax/A/V value achievable with conventional EO pump devices (i.e., ˜1-1.5 ml/(min V cm²)). At the same time, the disclosed EO pump devices use a minimal applied electric potential to reduce the electrolysis, electrode oxidation and Joule heating effects, and increase the thermodynamic efficiency.

APPLICATIONS

There are a variety of applications for EO pump devices. The core structure for the membrane and electrodes can be adapted to function with other pump components such as, for example, fluid chambers, inlet port(s), and outlet port(s), as known in the art.

These applications include, for example, lab-on-a-chip devices and applications, inkjet printing, ink delivery, drug delivery, liquid drug delivery, chemical analysis, chemical synthesis, proteomics, healthcare related applications, defense and public safety applications; medical applications, pharmaceutical or biotech research applications, environmental monitoring and defense applications, in vitro diagnostic and point-of-care applications, and medical devices. EO pump devices of the embodiments disclosed herein may also be incorporated into an inkjet printing device. Other application include PCR (DNA amplification, including real time PCR on a chip), electronic cooling (e.g., for microelectronics), using EO pump devices as valves by opposing pressure driven flow, using EO pump devices to fill and empty flexible reservoirs to induce functionality via shape change. Still other applications include, for example, pumping ionized fluids and colloidal particles, heat transfer and electronic cooling, adaptive microfluidic mirror arrays, EO-actuators, and EO valves. In another application, pores can be coated with certain chemicals, enabling chemical reactions and synthesis of new materials. A benefit for at least one of the embodiments is high throughput screening and compound profiling.

Multiple EO pump devices can be used together in series or parallel. The EO pump devices can be also integrated within micro-meter and millimeter scale fluidic systems, by, for example, stacking them together, for example, to increase the pressure build-up, or to maintain flow rate to overcome the viscous losses and pressure loads in long channels.

The devices described herein can be run on small watch batteries, and can thus enable a variety of hand held devices.

Electroosmotic pumps and their applications are known and can be adapted for the EO pump devices provided herein. See, for example, U.S. Pat. Nos. 7,667,319; 7,645,368; 7,274,106; 7,231,839, 7,185,697, 7,149,085, 7,134,486, 7,131,486, 7,105,382, 7,086,839, 7,084,495, 7,037,416, 6,992,381, 6,991,024, 6,942,018, 6,934,154, 6,861,274, 6,805,841, 6,747,285, 6,726,920, 6,639,712, 6,613,211, 6,595,208, 6,541,021, 6,395,106, 6,323,042, 6,315,940, 5,573,651.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.

Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 

1. An electroosmotic device comprising: a substrate comprising at least one pore extending from a first major surface of the substrate to a second major surface of the substrate, the at least one pore comprising one of a macropore, a micropore, or a nanopore; at least one first material disposed on at least the inner surface of the at least one pore, wherein the at least one first material is electrically conductive, at least one second material disposed at least on the at least one first material in the at least one pore, wherein the at least one second material is an electrical insulator, at least one anode electrode adjacent to the first major surface; and at least one cathode electrode adjacent to the second major surface.
 2. The device according to claim 1, wherein the at least one first material is in electrical communication with a first voltage source, independently biased from a second voltage source applying a voltage difference between the at least one cathode electrode and the at least one anode electrode.
 3. The device according to claim 1, the device further comprising a pump housing and at least two fluid chambers to provide inlet flow and outlet flow.
 4. The device according to claim 1, wherein the substrate comprises aluminum oxide or silicon.
 5. The device according to claim 1, wherein the at least one first material comprises at least one of metal, a metal alloy, a semiconductor, a conducting metal nitride, or a conducting oxide.
 6. The device according to claim 1, wherein the first material comprises at least one of Ti, Au, Pt, Al, Cu, Ag, W, nitride thereof, or ZnO
 7. The device according to claim 1, wherein the at least one second material comprises an oxide or metal oxide.
 8. The device according to claim 1, wherein the at least one second material comprises an oxide selected from the group consisting of HfO₂, ZrO₂, Al₂O₃, TiO₂, or SiO₂.
 9. The device according to claim 1, wherein the at least one first material has a thickness of at least 10 nm.
 10. The device according to claim 1, wherein an aspect ratio of the at least one pore is about 5 to about 1,200.
 11. The device according to claim 1, wherein the substrate is about 10 microns to about 200 microns thick.
 12. The device according to claim 1, wherein the at least one pore has a pore size that is between about 50 nm and about ten microns.
 13. The device according to claim 1, further comprising at least one electrically conductive mesh adjacent to the substrate and defining at least one of the at least one cathode electrode or the at least one anode electrode.
 14. A method of preparing an electroosmotic device comprising: obtaining a substrate comprising a first major surface, a second major surface, and at least one pore extending from the first major surface to the second major surface, the pore comprising one of a macropore, a micropore, or a nanopore; forming at least one first material on at least the inner surface of the at least one pore, wherein the at least one first material is electrically conductive; depositing at least one second material on at least one first material, wherein the second material is an electrical insulator; and providing at least one anode electrode adjacent to the first major surface and at least one cathode electrode adjacent to the second major surface.
 15. The method of claim 14, wherein the step of forming comprises providing a thickness for the at least one first material of at least about 10 nm.
 16. The method of claim 14, wherein the step of forming comprises depositing at least one electrically conductive material on the at least the inner surface of the pores to define the at least one first material.
 17. The method of claim 16, wherein the at least one first material is deposited by atomic layer deposition (ALD).
 18. The method of claim 17, wherein the ALD is carried out at a deposition temperature of about 100° C. to about 350° C.
 19. The method of claim 17, wherein ALD is carried out for at least 300 ALD cycles.
 20. The method of claim 17, wherein the ALD is carried out with a dwell time adapted to avoid blocking the pores but provide conformal coverage.
 21. The method of claim 14, wherein the at least one second material is deposited by atomic layer deposition (ALD).
 22. The method of claim 14, wherein the step of providing the substrate further comprising selecting the substrate such that the at least one pore has an aspect ratio of about 300 to about 1,200.
 23. The method of claim 14, wherein the step of forming comprises implanting at least one dopant into the at least the inner surface of the pores to define the at least one first material.
 24. The method of claim 14, wherein the step of providing further comprises positioning at least one electrically conductive mesh adjacent to the substrate to define at least one of the at least one cathode electrode or the at least one anode electrode.
 25. A method for using an electroosmotic device comprising: providing an electroosmotic device comprising a substrate having at least one pore that is a macropore, a micropore, or a nanopore, at least one first material formed on the inner surface of the at least one pore, at least one second material disposed on the at least one first material, at least one anode electrode adjacent to a first major surface of the substrate, and at least one cathode electrode adjacent to a second major surface of the substrate, wherein the at least one first material is electrically conductive and the at least one second material is an electrical insulator; applying a first voltage across the anode and cathode to generate electroosmotic flow through the at least one pore; and applying a second voltage independently biased from the first voltage to the at least one first material to modify the electroosmotic flow.
 26. The method of claim 25, wherein the modification of electroosmotic flow is an increase in flow rate, a decrease in flow rate, or a reversal of flow.
 27. The method of claim 25, wherein the step of applying the first voltage comprises selecting the first voltage to be five volts or less.
 28. An electroosmotic device comprising: a substrate comprising at least one pore extending from a first major surface of the substrate to a second major surface of the substrate, the at least one pore comprising one of a macropore, a micropore, or a nanopore; a plurality of dopant atoms embedded into at least the inner surface of the at least one pore to define at least one first material at the at least the inner surface of the at least one pore, the at least one first material being electrically conductive; at least one second material disposed at least on the at least one first material in the at least one pore, wherein the at least one second material is an electrical insulator, at least one anode electrode adjacent to the first major surface; and at least one cathode electrode adjacent to the second major surface.
 29. The device according to claim 28, wherein the at least one first material is in electrical communication with a first voltage source, independently biased from a second voltage source applying a voltage difference between the at least one cathode electrode and the at least one anode electrode.
 30. The device according to claim 28, the device further comprising a pump housing and at least two fluid chambers to provide inlet flow and outlet flow.
 31. The device according to claim 28, wherein the substrate comprises aluminum oxide or silicon.
 32. The device according to claim 28, wherein the plurality of dopants comprise p-type or n-type dopants.
 33. The device according to claim 28, wherein the at least one second material comprises an oxide or metal oxide.
 34. The device according to claim 28, wherein the at least one second material comprises an oxide selected from the group consisting of HfO₂, ZrO₂, Al₂O₃, TiO₂, or SiO₂.
 35. The device according to claim 28, wherein the at least one first material has a thickness of at least 10 nm.
 36. The device according to claim 28, wherein an aspect ratio of the at least one pore is about 5 to about 1,200.
 37. The device according to claim 28, wherein the substrate is about 10 microns to about 200 microns thick.
 38. The device according to claim 28, wherein the at least one pore has a pore size that is between about 50 nm and about ten microns.
 39. The device according to claim 28, further comprising at least one electrically conductive mesh adjacent to the substrate and defining at least one of the at least one cathode electrode or the at least one anode electrode. 